Modern vehicles include systems having complex electric circuits for performing different functions. Common circuits include an analog-to-digital converter (“ADC”) in communication with a sensor. The sensor measures a physical quantity and generates an analog electrical signal indicative of the measured physical quantity. The ADC receives the analog signal from the sensor and converts it into a digital electrical signal (i.e., a digital value, a digital output code, etc.).
The analog sensor signal is typically a voltage signal (i.e., sensor input VIN). The ADC generates the digital output code for the sensor input VIN by comparing the sensor input VIN with a voltage reference VREF. The voltage reference VREF may be externally provided to the ADC or may be internally generated in the ADC itself. The voltage reference VREF is intended to be a precise ‘measuring stick’ against which the sensor input VIN is compared. Under error free operation, the ADC generates the digital output code for the sensor input VIN according to the following equation:output=VIN*(2n/VREF)where “output” is the digital output code in decimal form and “n” is the number of bits of resolution of the ADC. The resolution indicates the number of discrete values the ADC can produce over the range of analog values. The values are usually stored in binary form so the resolution is expressed in bits. For example, an ADC with a resolution of eight bits can encode the analog sensor input VIN to one in 256 different levels since 28=256.
As shown in the above equation for generating the digital output code, the digital output code varies inversely proportional to the voltage reference VREF. Significant to embodiments of the present invention, as described below, is that the digital output code is a function of the voltage reference VREF. As such, the voltage reference VREF has to be precise in order for the ADC to output an accurate digital output code for a given sensor input VIN.
ADCs drift with aging. These drifts are directly proportional to the variation of the voltage reference VREF used by the ADC in the conversion process. Drifts due to aging are a relatively larger problem than absolute accuracy. The internal error can be calibrated, but compensating for drift is difficult. Where possible, voltage references should be chosen for aging characteristics which preserve adequate accuracy over the expected lifetime of the system employing the ADC.
Some systems such as battery monitoring systems in electric vehicles have requirements that overstress ADCs. These requirements include relatively long lifetimes (for example, a lifetime ten times longer than the lifetime of a conventional vehicle) and relatively high accuracy requirements over time.